1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber in which an optical fiber is manufactured by drawing a bare optical fiber from an optical fiber preform in an optical fiber manufacturing process.
2. Description of the Related Art
FIG. 14 is a schematic configuration diagram illustrating an optical fiber manufacturing apparatus used in manufacturing a general silica optical fiber. Manufacturing of an optical fiber in the conventional technique is performed through the following processes using the optical fiber manufacturing apparatus shown in FIG. 14.
(I) An optical fiber preform 101 made from a glass rod which is formed of material of an optical fiber is introduced into a heating furnace 102. Then, a leading edge of the optical fiber preform 101 is heated and melted at a temperature of approximately 2000° C. by a heater 102a, and the melted portion is drawn downward from the heating furnace 102 as a bare optical fiber 103.
(II) Next, the drawn bare optical fiber 103 is cooled. A cooling device 104 including a vertically long cooling tube is installed under the heating furnace 102. A cooling gas (helium gas (He) or the like) is supplied to the inside of the cooling tube from a side portion of the cooling tube. The cooling gas flows upward and downward inside the cooling tube. Arrow indications in the cooling device 104 in FIG. 14 indicate the flow 110 of the cooling gas. The bare optical fiber 103 drawn out of the heating furnace 102 is sufficiently cooled down to the temperature at which the bare optical fiber 103 can be coated by the coating resin.
(III) Next, a coating resin is coated on the cooled bare optical fiber 103 by a coating device 106 in order to protect the surface of the bare optical fiber 103. The coating resin is thermally cured by a curing device 108 or is ultraviolet-cured. Thus, the bare optical fiber 103 becomes an optical fiber 107. This coating resin is generally configured as a bi-layer structure, in which a material having a low Young's modulus is coated on the inner side and a material having a high Young's modulus is coated on the outer side.
(IV) Next, the optical fiber 107 is rolled up. The coated optical fiber 107 is rolled up by a winding device (not shown) through a turn pulley 109.
In the optical fiber manufacturing method, it is desirable to enhance the productivity of the optical fiber, to increase the size of the optical fiber preform, and to increase the fiber drawing velocity (hereinafter, referred to as “drawing velocity”).
In general, according to an acceleration in drawing velocity the amount of fluctuation of the drawing velocity (fluctuation of the range of center drawing velocity±X (m/min)) (hereinafter, referred to as “drawing velocity fluctuation range”) increases. For this reason, it is necessary to reliably perform protection coating (resin coating) with a constant coating diameter, in a large drawing velocity fluctuation range.
In a case where the drawing velocity is slow, for example, in a case where the drawing velocity fluctuation range is ±30 (m/min) at a drawing velocity of 300 (m/min), in this drawing velocity and drawing velocity fluctuation range, it is possible to reliably coat the bare optical fiber with a constant coating diameter with the coating resin, without using a particularly complicated process.
On the other hand, in a case where the drawing velocity is high as described above, for example, in a case where the drawing velocity is 2000 (m/min), if the drawing velocity fluctuation range is 10% (±200 (m/min)) of the drawing velocity in a similar way to the case where the drawing velocity is slow, the drawing velocity range becomes 2000 (m/min)±200 (m/min). Thus, it is necessary to reliably coat the bare optical fiber with a constant coating diameter with the coating resin in this drawing velocity range. However, in a case where the optical fiber manufactured at the same drawing velocity as in the case where the drawing velocity is slow, that is, in the drawing velocity range of 2000 (m/min)±30 (m/min) is used as a good quality portion, if the optical fiber is manufactured in the drawing velocity range of 2000 (m/min)±200 (m/min), faulty portions in the optical fiber significantly increase, and the yield ratio is decreased.
Further, as the drawing velocity increases, the length of the optical fiber required for a speed increase or speed stabilization increases until the drawing velocity reaches a high speed state (for example, about 2000 (m/min) or higher) which is the final drawing velocity from a slow speed state (for example, about 30 (m/min) or lower or higher) when the fiber drawing is started. As a result, the number of faulty portions increases in the manufactured optical fiber, and the yield ratio is decreased.
Further, if the fiber drawing is ended in a state where the drawing velocity is high, the coating device may be damaged by the terminal of the cut optical fiber. In addition, when the terminal of the optical fiber reaches an optical fiber winding bobbin, the terminal of the optical fiber strikes the good quality optical fiber, thereby damaging it.
For this reason, in the optical fiber manufacturing method, it is necessary to maintain the drawing velocity as slow as possible when the fiber drawing from the optical fiber preform starts, to maintain the manufactured optical fiber in a good quality state, and to reach the final drawing velocity in the good quality state. Further, in a terminal end of the optical fiber preform, it is necessary to stop the fiber drawing from the optical fiber preform after the drawing velocity is slowly decreased in this good quality state. Furthermore, in this optical fiber manufacturing method, it is necessary to maintain a constant coating diameter.
As factors causing a change in the coating diameter, a change in temperature of the bare optical fiber when the coating resin is coated, a change in the shear velocity of the coating material in die lands inside the coating device, and the like are generally exemplary examples.
The temperature change in the bare optical fiber during coating is represented as change in the cooling capacity of the cooling device when the bare optical fiber drawn out of the optical fiber preform is sufficiently cooled down to the temperature at which the bare optical fiber can be coated by the coating resin, in the drawing velocity range. This change in the cooling capacity significantly affects the change in the coating diameter. It is desirable to appropriately adjust the temperature of the bare optical fiber in a wide drawing velocity range.
The change in the shear velocity of the coating material in the die lands inside the coating device mainly depends on a viscosity change due to the change in the temperature of the coating material or a change in a coating material supply pressure into the coating device. However, the drawing velocity range insignificantly affects these changes, which may be considered as almost no effect.
Thus, an optical fiber manufacturing method is disclosed as described below.
There is disclosed a technique in which the total flow rate of two types of gases flowing into a cooling device is maintained at a constant value, the ratio of the flow rates of these gases is fed back to all of the gas lines according to the drawing velocity, and the temperature of the bare optical fiber or the coating diameter is maintained at a constant value (refer to Patent Document 1, for example). In this technique, the ratio of the gas flow rates is changed according to a signal indicating the temperature of the optical fiber or a signal indicating the coating diameter, instead of the drawing velocity. That is, this technique is a technique in which the feedback is basically applied to all of the gas lines from one signal line to maintain the coating diameter at a constant value.
In the technique disclosed in Patent Document 1, the gas flow rate of a specific amount or more is required to prevent mixture with outside air. Further, the mixture ratio of two or more types of gases which are used is changed while maintaining the total amount of the gas flow rates at a constant value in order to maintain the temperature of the bare optical fiber at a constant value with respect to a change in the drawing velocity (or the temperature of the bare optical fiber or the coating diameter).
In this way, in order to prevent the mixture of outside air, the gas flow rate of the specific amount or more is required. For this reason, if this technique is applied to a case where the optical fiber is adapted to a wide drawing velocity range or a case where the fiber drawing velocity is increased, the amount of used gas is significantly increased. Thus, the Reynolds number inside the cooling device is increased, and as a result, the flow of the gas becomes turbulent. Thus, the bare optical fiber inside the cooling device vibrates (fiber vibration), and thus, the coating becomes unstable. Further, in a case where fiber vibration is large, the bare optical fiber comes in contact with an inner wall of the cooling device to thereby damage the bare optical fiber, and thus, the strength of the manufactured optical fiber is lower, and thereby leads to breakage.
On the other hand, if the amount of the used gas is reduced, outside air is mixed into the cooling device making the cooling capacity unstable. As a result, the temperature of the bare optical fiber becomes unstable. For this reason, a large amount of gas is required, and thus, the usage amount of expensive helium gas increases, thereby increasing the manufacturing cost of the optical fiber.
Further, if the total amount of gas flow rates is controlled to be constant, other types of gas flow rates should be increased as one type of gas is reduced. In this case, it is difficult to obtain the total amount of gas flow rates at which the temperature of the bare optical fiber is maintained at a constant value.
Further, in a case where it is undesirable to set the flow rate of one type of gas to zero due to problems other than cooling (for example, prevention of the mixing of air bubbles into the coating resin, or the like) in order to maintain the temperature of the bare optical fiber at a constant value, an applicable drawing velocity range becomes narrow.
Further, it is difficult to change the density ratio of gases in the longitudinal direction inside the cooling device since the plurality of gases is mixed and then introduced into the cooling device, and to minutely adjust the cooling capacity. Thus, it is difficult to apply the technique to a wide drawing velocity range.
There is disclosed a technique in which two or more types of gases are introduced into a cooling device, in which these gases are divided into a gas having a fixed flow rate and a gas having a variable flow rate, feedback is applied using a signal indicating the coating diameter of an optical fiber, the flow rate of the gas having the variable flow rate is changed, and thus, the coating diameter is maintained at a constant value (refer to Patent Document 2, for example). This technique is a technique in which the feedback is applied by one signal line to maintain a constant coating diameter at a constant value.
In this technique, a specific amount of gas flow rate is required in order to prevent the mixture of outside air. Further, in a case where fiber drawing is performed at a high speed of 2000 (m/min) or higher, it is necessary to increase the flow rate of the gas having the fixed flow rate. For this reason, the bare optical fiber inside the cooling device is vibrated, thereby making the coating unstable. Further, in a case where the fiber vibration is large, the bare optical fiber comes in contact with an inner wall of the cooling device to thereby damage the bare optical fiber, and thus, the strength of the manufactured optical fiber is lower, and thereby leads to breakage.
Further, if the amount of the used gas is reduced, outside air is mixed into the cooling device, and thus, the cooling capacity becomes unstable. As a result, the temperature of the bare optical fiber becomes unstable. For this reason, a large amount of gas is required, and thus, the usage amount of expensive helium gas is increased, thereby increasing the manufacturing cost of the optical fiber.
Further, since a cooling gas having a high thermal conductivity is required to have the fixed flow rate, it is necessary to increase the flow rate of gas having a low thermal conductivity, in order to maintain the coating diameter of the optical fiber at a constant value in a wider drawing velocity range, in particular, in order to correspond to the case of a slow drawing velocity. For this reason, the bare optical fiber inside the cooling device is vibrated, thereby making the coating unstable. Further, in a case where fiber vibration is large, the bare optical fiber comes in contact with an inner wall of the cooling device to thereby damage the bare optical fiber, and thus, the strength of the manufactured optical fiber is lower, and thereby leading to breakage. Further, since helium gas having a fixed flow rate is present, even though the flow rate of gas having a lower thermal conductivity, as the gas having the variable flow rate, is increased, in a case where the drawing velocity is slow, the temperature of the bare optical fiber is decreased, thereby making it difficult to maintain the coating diameter at a constant value.
In the states of the slow drawing velocity and the high drawing velocity, as the drawing velocity becomes high, the flow rate of the gas having the low thermal conductivity has decreased and the flow rate of the gas having the high thermal conductivity has increased, inside the cooling device. As a result, when the flow rate of the gas having the low thermal conductivity inside the cooling device becomes zero, the cooling capacity of the cooling device is maximized. Thus, the gas flow rate is adjusted at the time of the state where the drawing velocity is slow, and thereafter, in a case where the drawing velocity is higher than or equal to the drawing velocity at the time when the flow rate of the gas having the low thermal conductivity becomes zero, the cooling device cannot sufficiently cool the bare optical fiber. As a result, there may be a case where the coating diameter of the coating resin is minutely changed, or in the worst case, the liquid coating material may be vaporized by the heat of the bare optical fiber, thereby causing coating errors.
In the techniques disclosed in Patent Document 1 and Patent Document 2, locations where the cooling device is in contact with outside air are present above and below. For this reason, if the gas flow rate or gas temperature of the cooling device and the fiber drawing velocity are changed, the gas flow may become unstable (upward stream and downward stream). That is, since the gas flow is changed in the slow drawing velocity range and the high drawing velocity range, the cooling capacity is significantly changed at the time of the gas flow change. Thus, in a wide drawing velocity range, it is difficult to maintain a constant cooling capacity of the cooling device at a constant value, and to maintain a constant temperature of the bare optical fiber or the coating diameter of the coating resin at a constant value.